Process for deposition of pyrolytic graphite



Oct. 15, 1963 R. J. DIEFENDORF 3,107,180

PROCESS FOR DEPOSITION OF PYROLYTIC GRAPHITE Filed Jan. 26, 1951 In Ve nto r'.- /Qusse .Die e recalca-'1 His A ttor-'T7 ey.

United States Patent Or 3,107,130 PRCESS FR DEPGS'HON 0F PYRLYTECGRAPHITE Russell I. Diefendcrf, Schenectady, N. assigner to GeneralElectric Company, a corporation of New York Fiied Jan. 26, 1961, Ser.No. 85,049 6 Claims. (Cl. 117-226) rI'his invention relates todeposition methods of forming articles and coatings and moreparticularly to deposition methods of forming uniform, dense pyrolyticgraphite articles and coatings.

Pyrolytic graphite is defined as a polycrystalline material made fromcarbonaceous gases by thermal decomposition or from a carbonaceousmaterial by evaporation and deposition on a surface in which the planargraphite crystallites are aligned into a layer structure. It is usefulas a high temperature material for lamp filaments, furnace linings andneutron reactor moderators. Development of missile and space propulsionsystems has created an additional requirement for pyrolytic graphitecomponents in these systems.

Carbonaceous gases have been thermally decomposed into carbon vapor anddeposited on surfaces to produce pyrolytic graphite. As a result of thedecomposition, carbon is removed from the gas and deposits on thesurface so that planar graphite crystallites are aligned into a layerstructure. It is desirable to provide uniform, dense pyrolytic graphitearticles at high deposition rates. Since such high rates are desirable,it would appear that gas pressure or temperature could be increased inthe deposition chamber to produce a corresponding increase in depositionrate. However, a uniform deposition at an increased rate depends upon anumber of variables. rl`hus, a mere increase in pressure or temperaturedoes not solve the deposition problernhutV imposes a subsequentlimitation by creating soot in the carbon vapor. Therefore, it would bedesirable to provide deposition methods of forming pyrolytic graphitewithout soot particles at an increased rate of deposition.

It is an object of my invention to provide an improved deposition methodof forming pyrolytic graphite articles and coatings.

it is another object of my invention to provide a deposition method offorming pyrolytic graphite articles and coatings in which non-sootingcarbon vapor is employed.

It is a further object of my invention to provide a deposition method offorming pyrolytic graphite articles and coatings in which a sootinhibiting gas is employed.

It is a still further object of my invention to provide a depositionmethod of forming pyrolytic graphite rticles and coatings with similarproperties and at high rates of deposition.

In carrying out my invention in one form, at least one member ispositioned within an enclosure, a carbonaceous material is fed to theenclosure, 0.1 volume percent to 5.0 volume percent nitric oxide ismixed with the material, the mixture is heated to decompose into anon-sooting carbon vapor, and the carbon vapor is flowed through theenclosure to form pyrolytic graphite on the member.

These and various other objects, features, and advantages of theinvention will be better understood from the following description takenin connection with the accompanyin g drawing in which:

FIGURE l is a sectionalview of a deposition apparatus for formingarticles in accordance with my invention; and

FIGURE 2 is a sectional view of a modified deposition apparatus.

In FiGURl-E 1, a deposition apparatus is shown generally at 10 in whicha member 11 in the form of a hollow 3E., E Patented Oct. 15, 1963 icegraphite cylinder is positoned within an enclosure 12 including aninsulated cylinder 13 of quartz or alumina, and carbon black insulation14. Member 11 is provided with an opening 15 in its bottom wall whichcommuni- Cates with the outlet of a preheater 16 and with an opening 17in its top wall which communicates with an outlet line 1S and associatedvacuum pump 19. Insulation 14 surrounds substantially member 11. Heatingcoils 20 surround cylinder 13 to provide heat for member 11 andpreheater 16. The inlet of preheater 16 is connected to an inlet line 21which branches into lines 22 and 23. A carbonaceous gas is fed from acarbonaceous material source (not shown) through line 22 including atotal consumption gas meter 24, a gas rate meter 25, and an ace'- toneand Dry lce trap indicated at 26 inlet line 21, and preheater inlet topreheater 16. While a carbonaceous gas, such as methane or ethane, isemployed, the carbonaceous material can also be in liquid or solid formwhich is fed from the source to preheater 16 for conversion to a carbonvapor. Second line 23 feeds nitric oxide, a soot inhibiting gas, througha total consumption meter 27 and a gas rate meter 2S from a gas source(not shown) to inlet line 21. The carbonaceous material and nitric oxideare mixed together in line 21 and ow into preheater 16 where additionalmixing occurs. If it is desired, lines 22 and 23 can be connecteddirectly to preheater 16 to cause mixing therein.

In FIGURE 2 of the drawing, a modified deposition apparatus is shownwhich comprises an enclosure having a lower body portion 31 and a cover32 which is hinged to lthe lower body portion by means of bolts andemploys an O ring 34 therebetween. Viewing window 35 is provided incover 32 to View the operation and to read an optical pyrometer (notshown). A line 36 extends through the bottom wall of enclosure 30 and isconnected to a preheater 37 which consists of a container 3S having aninlet and an outlet. A bafiie AE59 is positioned container 38 and -aplurality of openings 40 are located near the perimeter thereof. 'Iheinlet of preheater 37 is connected to an inlet line 41 which branchesinto lines 42 Iand 43. A carbonaceous gas is fed from a carbonaceoussource (not shown) through line 42 including tot consumption meter 24,gas rate meter 25, and an acetone and Dry Ice trap 26, Vinlet line 41,and preheater inlet to preheater 37. Second inlet line 43 feeds nitricoxide through total consumption meter 27 and gas rate meter 2S from agas source to inlet line 41 (not shown). The carbonaceous material andnitric oxide are mixed together in line 41 and flow into preheater 37where additional mixing occurs. A member 44 in the form of a sheet `ofgraphite is suspended by any suitable means, such as a hook 45 and 4arod 46 within enclosure 30. An outlet 47 is provided in enclosure 30 towhich is connected a line 43 associated with a vacuum pump 49 to reducethe chamber pressure to a desired level. Heat is supplied to preheater37 from any suitable source, such as heating coils (not shown).

I discovered unexpectedly that pyrolytic graphite articles and coatingswere formed uniformly and without soot particles at high volumecarbonaceous gas flow rates by positioning at least `one member withinan enclosure, feeding a carbonaceous material to the enclosure, mixing0.1 volume percent Ito v5.0 volume percent of nitric oxide, a sootinhibiting gas, with the material, heating the mixture to decompose themixture to a non-sooting carbon vapor, and flowing the carbon vaporthrough the enclosure. I found further that the preferred range ofnitric oxideL was 0.1 volume percent to 5.0 volume percent. Within thisran-ge, the most desirable addition appeared to be 1.5 volume percent.Additionally, I found that this deposition method was applicable over -aWide temperature range, such as 1000`cJ C. to 2300 C.

sootng carbon vapor.

-It appears that the soot particle is probably formed by the growth of`a large carbon molecule which upon reaching a critical size is coatedaround its periphery with smaller 'aromatic molecules. These smallermolecules are oriented with their basal planes parallel to the surfaceof the larger molecule. If a large diameter chamber is ernployed lat lowpressure, the large carbon molecule does not have time to diff e to thechamber wall before it forms soot. An increase in pressure increases thesoot formation. -If a small diameter chamber is used at low pressure,the large carbon molecule does have time to diffuse to the chamber wallwithout soot formation. However, a large increase in pressure in thesmall diameter chamber to'increase deposition produces soot rather thana'uniform, dense pyrolytic graphite article or coating.

In the operation -of deposition apparatus shown in FIGUREl 1, cylinder13 is placed on a surface and surrounded by coils 20. Member 11 withpreheater 16 is positioned within cylinder 13 Kand placed on thesurface. The void around and beneath lmember 11 is filled with carbonblack 14. Outlet line 18 is connected to opening 17 in member 11 yandwith its associated pump 19. Additional insulation is placed on top ofmember 11 to complete enclosure 12. Inlet line 20 with its associatedbranch lines 21 and 22 is connected to the preheater inlet. Acarbonaceous gas, such as methane, is fed through line 22 includingytotal consumption meter 24, gas rate meter 25 and acetone and Dry Icetrap '26, inlet line 21, and preheater inlet to preheater 16. Nitricoxide is fed through line 23 including total consumption meter 27 andgas rate meter 28 to inlet line 21. The carbonaceous material `andnitric oxide are mixed together in line 21 and ow into preheater 16where additional mixing occurs. The mixture is heated in preheater 16 todecompose the mixture to form a non-sooting carbon vapor. I have foundthat a wide temperaturerange can be employed to decompose ythecarbonaceous material and nitric oxide mixture. Since the rate ofdeposition of such a carbon vapor yon a surface to form a pyrolyticgraphite article is increased by increasing the temperature, it ispreferred to heat the mixture in a temperature range between 1000" C.and 2300 C. Generally, maximum deposition occurs in a temperature rangeof greater than 2000 C.

When soot forms during the heating of a carbonaceous material, it formsheavily in the preheater and continues its formation in the depositionchamber. 0.1 volume percent to 5.0 volume percent of nitric oxide ismixed with the carbonaceous material and the mixture is heated todecompose into a non-sooting carbon vapor in the pret heater.Approximately 1.5 volume percent of nitric oxide mixed with -acarbonaceous material appeared to be the optimum vamount of sootinhibiting gas to produce a non- -Increased amounts of nitric oxideproduced only small additional effects. A wide pressure range can beemployed since a non-sooting carbon vapor is used. While a pressurerange, such as 0.5 mm. to 760 mm. of mercury -at various gas ilow rateslis desirable, I prefer to reduce the chamber latmosphere to a pressureof 0.5 toy 80 mm. of mercury lat gas flow rates of 8 to 150 cubic feetper hour.

As shown in FIGURE l of the drawing, non-sooting carbon vapor which hasbeen formed in preheater 16 is fed through opening 1S into hollow member11 where it forms pyrolytic graphite on the inner wall of the rnernber.Line y1S, with its 'associated pump 19, maintains enclosure 12 at thedesired pressure. Since an increase in pressure produces generally anincrease in deposition rate, the required time period to produce thedesired article thickness can be reduced.

In the operation of the deposition apparatus shown in FIGURE 2, a member44 in the form of a sheet of graphite is suspended by hook 45 and rod 46Within enclosure 30. Cover 32 is bolted to lower body portion 31 ofenclosure and the enclosure atmosphere is reduced tov a pressure in therange of 0.5 mm. of mercury to 760 mm. of mercury. Heat is supplied topreheater 37 from any suitable source, such as heating coils (notshown).

A carbonaceous gas, such as methane, or a mixture of a carbonaceous gasand a reacting gas, such as hydrogen, is fed from a source (no-t shown)through line 42 including meter 24, meter 25, and trap 26, inlet line41, and preheater inlet to preheater 37. Nitric oxide is fed throughline 43 including meters 27 and ,2S from a gas source (not shown) toinlet line 4'1. The carbonaceous material and nitric oxide are mixedtogether in line 41 and flow into preheater -37 where additional mixingoccurs. The mixture is heated inpreheater 37 to decompose the mixture toa non-scoring carbon vapor. As in the apparatus in FIGURE l, a widetemperature range can be employed in the apparatus of FIGURE 2 todecompose the carbonaceous material and nitric oxide mixture. It ispreferred to heat the mixture in a temperature range between 1000" C.and 2300 C. 0.1 volume percent to 5.0 volume percent of nitric oxide ismixed with the carbonaceous material and the mixture heated to thedesired temperature without soot formation in the preheater. A widepressure range, as set forth above for apparatus 10 in FIGURE l,can'also be employed.

As shown in FIGURE sooting carbon vapor which has been formed inpreheater 37 is fed through line 36 to enclosure 30 where it tonuspyrolytic graphite on member 44 suspended within enclosure 30. Line 48,with associated pump 49, maintains enclosure 30 at the desired pressure.Since an increase in pressure produces generally an increase indeposition rate, the required time period to produce the desired articlethickness can be reduced.

An example of an attempted method of :forming a pyrolytic graphitearticle without the inclusion of nitric oxide is set lforth below inExample I.V Examples II and III are deposition methods of kformingpyrolytic graphite in accordance with the present invention.

Example I A deposition apparatus was set up generally in accordance withFIGURE 1 of the drawing wherein a member in the form of a hollowcylinder was composed of cornmercial graphite. The enclosure atmospherewas reduced to a pressure of .001 mm. of mercury by the pump. Power wassupplied to the induction coil to heat the enclosure and member to anapproximate temperature of about 20110 C. A carbonaceous gas in the formof methane was supplied at a rate of 60 cubic feet per hour at `apressure of 20 mm. of mercury to the preheater through metering devices,and an on the member as it owed through the enclosure. After Example llAfter the sooty coating was formed in Example L Example III After thesooty coating was formed subsequent to stopping lthe nitric oxide flowin Example II, the tempera.- ture in the enclosure was reduced to l900C. A soot inhibiting gas in the form of nitric oxide was supplied at arate of one cubic foot per hour to the preheater sub- 2 of the drawing,the non- I sequent to flowing through metering devices. 'I'he gases weremixed -in the preheater. The gases were heated in the preheater andformed a non-sooting carbon vapor in the preheater which vapor wasdeposited on the member as a non-sooty coating as it owed through theenclosure. After two minutes, the nitric oxide tlow was stopped and asooty coating was again formed on the member. The power and gas flowwere discontinued and the chamber was restored to atmospheric pressure.

While other modifications of this invention and variations of methodwhich may be employed within the scope of the invention have not beendescribed, the invention is intended to include such that may beembraced Within the following claims.

What I `claim as new and desire to secure by Letters Patent of theUnited States is:

1. A deposition method which comprises providing an enclosure,positioning at least one member within said enclosure, heating acarbonaceous material to decompose said `carbonaceous material to acarbon vapor, mixing 0.1 volume percent to 5 .0 volume percent of nitricoxide with said carbon vapor to produce a non-scoring carbon vapor, andowing said non-sooting ycarbon vapor through said enclosure wherebypyrolytic graphite is yformed on said member.

2. A deposition method which comprises providing an enclosure,positioning at least one member within said enclosure, heating acarbonaceous material to a ternperature in the range of 1000 C. to 2300C. to decompose said carbonaceous material to a carbon vapor, mixing 0.1volume percent to 5.0 Volume percent of nitric oxide with said can-bonvapor to produce a non-sooting carbon vapor, and owing said non-sootingcarbon vapor through said enclosure whereby pyrolytic graphite is formedion said member.

3. A deposition method which comprises providing an enclosure,positioning at least one member within said enclosure, yfeeding acarbonaceous gas to said enclosure,

mixing 0.1 volume ypercent to 5.0 volume percent of nitric oxide withsaid rst gas, heating said mixture to decompose said mixture to anon-sooting carbon vapor, and owing said carbon vapor through saidenclosure whereby pyrolytic graphite is formed on said member.

4. A deposition method which comprises providing an enclosure,positioning at least one member within said enclosure, heating a`carbonaceous material to decompose said carbonaceous material to acarbon vapor, mixing 1.5 volume percent of nitric oxide with said carbonvapor to produce a non-sooting carbon vapor, and Flowing saidnon-sooting carbon vapor through said enclosure whereby pyrolyticgraphite is formed on said member.

5. A deposition method which comprises providing an enclosure,positioning at least one member Within said enclosure, feeding methanegas to said enclosure, mix ing 1.5 volume percent of nitric oxide withsaid methane gas, heating said mixture at a temperature in the range of1000 C. to 2300 C. to decompose said mixture to a non-sooting carbonvapor, and owing said carbon vapor through said enclosure wherebypyrolytic graphite is rformed on said member.

6. A deposition method which comprises providing an enclosure,positioning at least one member within said enclosure, mixing acarbonaceous gas and 0.1 volume percent to 5.0 volume percent of nitricoxide, heating said mixture to decompose said mixture to a non-sootin-gcarlbon vapor, and flowing said carbon vapor through said enclosurewhereby pyrolytic graphite is formed on said member. i

References Cited in the le of this patent UNITED STATES PATENTS

1. A DEPOSITION METHOD WHICH COMPRISES PROVDING AN ENCLOSURE,POSITIONING AT LEAST ONE MEMBER WITHIN SAID ENCLOSURE, HEATING ACARBONACEOUS MATERIAL TO DECOMPOSE SAID CARBONACEOUS MATERIAL TO ACARBON VAPOR, MIXING 0.1 VOLUME PERCENT TO 5.0 VOLUME PERCENT OF NITRICOXIDE WITH SAID CARBON VAPOR TO PRODUCE A NON-SOOTING CARBON VAPOR, ANDFLOWING SAID NON-SOOTING CARBON VAPOR THROUGH SAID ENCLOSURE WHEREBYPYROLYTIC GRAPHITE IS FORMED ON SAID MEMBER.